A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is desirable to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Many types of metrology (inspection) devices produce results that are dependent on the wavelength of the radiation used, for example because layers on the substrate being inspected have wavelength dependent optical properties or because layer thicknesses or structural pitches differently diffract the radiation used. Thus inspection devices of various types use either multiple monochromatic sources (e.g. lasers) providing outputs of different wavelengths or broadband sources and selectively insertable filters to enable measurements at different wavelengths to be performed either simultaneously or selectively.
However, coupling multiple laser sources into a single optical system of an inspection device often requires expensive optical multiplexing and/or demultiplexing devices and may be sensitive to alignment of the sources and other optical components. Swapping filters into and out of the inspection beam can be time consuming, reducing throughput of the inspection device when multiple measurements at different wavelengths need to be made.
An ellipsometer using a broadband source and an acousto-optical filter for fast wavelength selection is disclosed in WO 95/17662.